1. Field of the Invention
The present invention relates to fuel injectors for high temperature applications, and more particularly, to fuel injectors for gas turbine engines.
2. Description of Related Art
Nozzles for injecting fuel into the combustion chamber of gas turbine engines are well known in the art. U.S. Pat. No. 6,688,534 to Bretz, which is incorporated by reference herein in its entirety, describes several aspects of fuel nozzles for gas turbine injectors. Fuel injectors for gas turbine engines on an aircraft direct fuel from a manifold to a combustion chamber of a combustor. The fuel injector typically has an inlet fitting connected to the manifold for receiving the fuel, a fuel nozzle located within the combustor for spraying fuel into the combustion chamber, and a housing stem extending between and fluidly interconnecting the inlet fitting and the fuel nozzle. The housing stem typically has a mounting flange for attachment to the casing of the combustor.
Fuel injectors are usually heat-shielded because of high operating temperatures arising from high temperature gas turbine compressor discharge air flowing around the housing stem and nozzle components. The heat shielding prevents the fuel passing through the injector from breaking down into its constituent components (i.e., “coking”), which may occur when the wetted wall temperatures of a fuel passage exceed 400° F. The coke in the fuel passages of the fuel injector can accumulate and restrict fuel flow to the nozzle.
The compressor air flowing through a fuel injector can reach temperatures as high as 1600° F. Heretofore, injector nozzles have included annular stagnant air gaps as insulation between external walls, such as those in thermal contact with high temperature ambient conditions, and internal walls in thermal contact with the relatively cool fuel. These insulative air gaps are generally open to the ambient conditions to allow for relative thermal expansion of injector components. When the engine is not in operation, fuel can be drawn into the insulative air gaps, and when the engine is subsequently operated, this fuel in the insulative gaps can coke and thereby reduce the insulative effects of the heat shielding. Thus cleaning of the fuel injector is required to prevent reduced thermal insulation, potential carbon jacking and diminished nozzle service life.
Although some solutions to this problem have been developed, such as in U.S. Pat. No. 5,761,907 to Pelletier et al., which describes attaching the inner heat shield to the downstream tip of the injector while leaving the upstream end free for thermal expansion, there are disadvantages to leaving the upstream end of the heat shield free. Among the disadvantages are potentially severe failure effects that can be caused by a fuel leak in the insulative gap allowing fuel to flow out of the upstream vent into an undesirable area of the engine, e.g. upstream of the nozzle. Therefore, it is common practice to locate the vent downstream near the fuel exit of the nozzle. With the vent opening downstream near the nozzle exit, in the event of a failure causing an internal fuel leak, fuel can be directed to flow out of the vent and into the combustor downstream. This allows for further albeit limited engine operation until the injector can be replaced. Therefore it is desirable for the diametrical clearances between the heat shield and the fuel swirler to be located downstream, rather than upstream as described by Pelletier, et al.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for a nozzle or fuel injector that allows for differential expansion while reducing or preventing fuel entry into the insulative gaps. It is desirable for such a nozzle to vent the insulative gaps downstream rather than upstream in the nozzle. There also remains a need in the art for such a nozzle or injector that is inexpensive and easy to make and use. The present invention provides a solution for these problems.